docs: def add_storage_account(self, name, key, endpoint):
          # type: (StorageCredentials, str, str, str) -> None
          """Add a storage account
          :param StorageCredentials self: this
          :param str name: name of storage account to store
          :param str key: storage key or sas
          :param str endpoint: endpoint
          """
          if name in self._storage_accounts:
              raise ValueError(
                  '{} already exists in storage accounts'.format(name))
          self._storage_accounts[name] = StorageAccount(
              name, key, endpoint,
              self._general_options.concurrency.transfer_threads,
              self._general_options.timeout,
              self._general_options.proxy,
          )